In a disc drive data is recorded on a disc in concentric, circular paths known as tracks. Servo bursts are written in each track on the disc and contain position information. The servo bursts are positioned along radial slightly wedged shaped quasi-lines that cross the circular tracks and divide the disc. The disc is formatted before user data is stored on the disc, and the format creates a number of sectors in the track area that lies between each pair of adjacent servo bursts. The number of sectors may vary from one track area to the next. During operation the disc continually rotates and a read/write head a given radius from the center of the disc reads or writes data in a given track. An actuator arm swings the head in an arc across the disc surface to allow the head to read or write data in different tracks.
Each sector located in each track area formed between the adjacent servo bursts contain several informational sections. With respect to FIG. 3-1, a track area between adjacent servo bursts is shown. The area contains two sectors with each sector containing several sections. The actual number of sectors between servo bursts is not limited to any specific number and will vary from drive to drive. These sections include a phase lock oscillator (PLO) section which provides the read and write parameters for the sector that are used by automatic gain control circuitry. A sync byte section is included to signal the beginning of a user data section where the application information is stored. An error correction code (ECC) section is typically provided after the user data section and permits correction of user data read back errors. After the ECC, a new section begins with another PLO section. This sector layout containing the several sections and the function and contents of each section are well known in the art.
The disc or magnetic storage media of the disc drive containing these sectors is susceptible to defects such as thermal asperities. The defects cause errors to occur during operation of the disc drive if the defect is of a significant size for the particular section or sections of the sector in which it lies. If the defect lies in the user data section, the ECC may be able to correct any resulting read error if the defect is not too large. If the defect lies within the ECC, the ECC's redundancy may account for any error. If the defect lies within the sync byte or the PLO, a relatively small defect may be enough to cause the sector to fail and the ECC cannot account for errors in the PLO and sync byte. If the disc drive electronics cannot properly set the gain and frequency for read back of the user data and cannot properly determine when to start the user data read back, invalid data will result.
Therefore, defect detection processes have been employed to find the defect and shift the sectors' positions within the track area so that the defect either lies in a section that allows the defect to be compensated for, such as the user data section or the ECC section. If the defect is too large in these sections, then the sector must be marked as unusable. This shifting process successfully avoids errors due to the defect, but such shifting processes require extra space to be dedicated within the servo bursts and this extra space reduces the storage capacity of the disc drive.
Furthermore, these defects in the storage media can grow over time. If the error is located near the end of the sector, future errors may result if that defect grows into the trailing sector. Even if the defect is initially large enough to force the sector to be marked as unusable, the defect may grow beyond the end of this unusable sector and may interfere with the PLO and sync byte of the trailing sector that was previously determined to be usable. This defect's growth into the trailing sector will cause the disc drive to eventually fail because the potential for growth of the defect has not been accounted for in the defect mapping process.